Reducing Reagent-Induced Activation of Adenylate Cyclase in the Cellular Slime Mold, Dictyostelium discoideum

Abstract

Binding of an intrinsic agonist (cAMP) to specific receptors on the cell surface induces transmembrane signals for activation and desensitization (adaptation and down regulation) of adenylate cyclase in the cellular slime mold, Dictyostelium discoideum. It is generally believed that dithiothreitol (DTT) induces the activation through interaction between the receptor and gradually accumulated cAMP, since DTT is known to inhibit cAMP-phosphodiesterase which degrades cAMP. In the present paper, we investigated the mechanism of activation of adenylate cyclase by the thiol-reducing agents, DTT and 2, 3-dimercapto-l-propanol (BAL). We found that BAL activated adenylate cyclase transiently even under conditions where the intrinsic agonist supersaturated the cAMP-receptors and competitively inhibited phosphodiesterase. This result is inconsistent with the generally accepted notion. We conclude that BAL has an independent effect from those of the intrinsic agonist (cAMP) and phosphodiesterase in activation of adenylate cyclase. Since BAL could induce activation just after the activation induced by a supersaturating concentration of the intrinsic agonist had ceased, the independent effect of BAL is not a simple enhancement of the cAMP-induced activation. Our result also suggests that the cAMP-induced adaptation (but not down regulation) suppresses the BAL-induced activation while BAL itself does not induce adaptation to cAMP or BAL. We propose that the thiol-reducing reagent induces or modifies the transmembrane activation signal for adenylate cyclase.